The Chinese company said that a Nigerian tax office had "raised a disagreement" relating to the tax filings for its 2006 purchase of a 45% stake in the licence from South Atlantic Petroleum (SAP).
CNOOC Ltd has contested the initial tax assessment, which reportedly followed the tax office's audit last year of SAP.
"After seeking legal and tax advice, the company's management believes we have reasonable grounds in making the contest," said CNOOC.
"Consequently,
